CHAPTER XIII. 



Synchronous Motors Method of synchronizing a Power Plant consisting of 

 Generator and Motor Armature Reaction Stability of Plant. 



SYNCHRONOUS MOTORS. 



81. Just as a direct- current generator can be driven as a motor by 

 applying a continuous P.D. between its terminals, so can an alter- 

 nator, if its field is separately excited, and a suitable alternating 

 P.D. be applied between its armature terminals. 



Suppose that the field of an alternator is excited by a direct 

 current in such a way that its poles are alternately north and 

 south, and suppose an alternating current is sent through its arma- 

 ture coils so that each armature pole is for one half-period of the 

 current a north pole, and for the next half-period a south pole. It 

 is easily seen that there will be impulses on the armature tending 

 to drive it first in one direction and then in the other, so that if the 

 armature is initially at rest, no motion at all will take place. But 

 suppose that a motion is mechanically given to the armature, so 

 that in half a period of the alternating current an armature pole 

 passes from one field pole to the next, then the impulses due to 

 the reaction between field and armature will always tend to drive 

 the armature in the same direction, and motion will continue so 

 long as no external influence causes the armature to revolve at a 

 different rate. 



An alternator driven as a motor can therefore only run at one 

 fixed speed, depending upon the periodicity of the current 

 which drives it. On this account, such a machine is called a 

 Synchronous Motor, since it will only run at that 

 particular speed which corresponds to synchronism with the 

 alternating current which drives it. 



